A two-stage thermostat is a programmable thermostat that monitors how long it takes a heating system to reach the set point and adjusts it to save energy. It prevents the furnace from firing up into its second stage function on cold days when it's not necessary.
The thermostat senses an increase in temperature of 6 to 8 degrees per hour. It then adjusts the start-up point at a fixed interval from the set point. It helps a furnace run at a lower output for a longer period of time. A two-stage thermostat allows the homeowner to have more control and improve energy efficiency.